1. Field of the Invention
The invention relates to a method of manufacture of a rotary anode assembly for an X-ray tube, and more particularly to a method wherein the base element consists of graphite or some other carbon based or ceramic material and the focal track is applied by a coating process.
2. Description of the Related Art
As a general rule, rotary anodes for X-ray tubes consist of a disk-shaped base element with an annular focal track coating of a high-melting-point metal or alloy, which generates the desired X-radiation by electron bombardment of the focal track coating.
The center area of the base element is connected to a cylindrical hollow metal shaft which is in turn connected to a rotor as the drive element of the rotary anode.
Rotary anode assemblies with a metal disk as the supporting base element usually have a central through-hole in the finish-machined rotary anode base element, into which the shaft is inserted and mechanically connected by a threaded fastening. In this way, a secure, sufficiently stable connection of these two mechanical components is achieved.
Rotary anodes must be accelerated in operation to a very high circumferential velocity within an extremely short period of time. For this reason, and particularly for large rotary anode dimensions, as required, in particular, for computer tomography, the heavy metal base elements are often replaced by those made of graphite or some other highly heat-resistant material based on carbon or ceramics with a lower specific gravity.
The advantage of the lower specific gravity of these materials as compared to metals with a comparable thermal capacity, however, is often accompanied by the disadvantage of lower strength, which can have a negative effect, in particular, regarding the connection between the rotary anode base element and the shaft.
Thus it is particularly disadvantageous that rotary anodes of low-density but weaker materials tend to burst when the base element is provided with a central through-hole for mechanical connection to the shaft. Such a connection of rotary anode and shaft is described, for instance, in U.S. Pat. No. 4,276,493. In order to eliminate this disadvantage, particularly when graphite is used as the base element, there have been suggestions to connect the shaft to the base element by soldering, without a through hole.
DE 24 25 082 A1, for instance, describes connecting rotary anode base elements to hollow shafts by welding and/or soldering. Among others, the connection of a base element made of graphite to the shaft is described. For the connection, the finish-machined base element with the focal track applied is inserted by a central cylindrical projection formed onto the bottom side into the tubular shaft and then the end of the projection is welded to the inside wall of the shaft. The formed projection, however, is not suitable to the material for strength reasons, even with large transition radii between base element and projection. Due to notch effects, material cracks can result, which can bring about a failure of the rotary anode in operation.
According to another example, in which the finish-coated and machined rotary anode base element with an end that is closed or expanded like a collar is butt-soldered to the tubular shaft, it is necessary to provide at least a centering aid in the form of a central depression in the rotary anode base element. Since the solder must be inserted between the contact surfaces in this type of soldering, a lateral displacement or tilting of the rotary anode base element with respect to the longitudinal axis of the shaft can often occur upon liquefaction of the solder during the soldering process, despite the centering aid. This leads to an eccentricity of the rotary anode in the radial or axial direction, which must be compensated for by mechanical machining after the soldering process. The compensation for an eccentricity in the axial direction is particularly costly for rotary anodes with a focal track applied by a coating process, since the coating must be applied to a corresponding excess dimension in order to permit a subsequent compensation of the eccentricity without the focal track becoming too thin at any one point. Since the material of the focal track is expensive, and the coating processes are inherently cost-intensive, any necessity to increase the layer thickness is a serious defect. Additionally, differing thicknesses of the focal track coating on the rotary anode cause a differing roughening behavior of the focal track, which is likewise undesired for usage.
For these reasons, it is desired to produce rotary anodes of highly heat-resistant materials with good strength such as, in particular, graphite, with a focal track coating applied by a coating process and without a central through-hole of the rotary anode base element.